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Aerodynamics

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Aerodynamics

Lecture 2

Chapter 2

- Balloons
- Cayley
- Wright Brothers
- Aerodynamics
- Da Vinci
- Montgolifer

LIFT

What is lift?

- Gravity
- The force that opposes lift is the pull of gravity.
- Gravity acts on all bodies on or near the surface of the Earth.
- The resulting force that gravity causes on some mass is referred to as weight.

- In order to rise into the air, a force must be created that is equal to or greater than the weight acting in the opposite direction.

- In the balloon example last class, the lift force was caused by an imbalance between pressure over the surface of the balloon and the weight of the balloon.
- With the balloon and its contents being lighter than the surrounding air, the imbalanced resulted in a net force in the upward direction.

- Airplanes are heavier than the air that they displace and must rely on other means to develop lift.
- What device is employed to develop lift on an airplane?

The Physics of Lift

The principle of lift discovered by a Swiss mathematician is?????

- He never heard of an airplane.
- His work had nothing to do with flight.
- He was trying to explain mathematically, the variation in pressure exerted by a moving mass of fluid ( a stream of water)

- His equation says that the pressure plus ½ times the density times the velocity squared must always equal a constant value.
- Pressure + ½ x density x velocity squared = constant.

- Static pressure + dynamic pressure = constant.
- Static means still (last lecture)
- Static pressure is used to describe the pressure exerted by a static, or still, air mass.
- Dynamic pressure is the pressure that would be exerted if the fluid were brought to rest.

- That fluid only contains so much pressure.
- If a fluid is a rest, all of the pressure is static pressure,
- however, if the fluid is in motion, then some static pressure is traded off for dynamic pressure.

- As the air flow speeds up, there is an increase in dynamic pressure,
- the static pressure must be reduced because the sum of the two pressures must be the same.

- Static pressure + dynamic pressure= total constant.
- 6+6 =12
- Now suppose the fluid speeded up.
- SP DP= TC
- 4+ 8 = 12
- Dynamic pressure increased so static pressure must decrease so the end result is constant (p.14).

- The law of continuity or continuity equation called the Law of Conservation of Matter.
- Applied to a moving stream,it states that in a moving stream of fluid, the density times the cross sectional area of the flow times the velocity must be a constant.
- Q x A V= constant.

- A venturi tube is a good example for Bernoulli’s principle because it has a restricted area of flow.
- Think about canoeing down a river. When is the water static?
- When is the water moving rapidly?

- A flow coming into a venturi tube will have an increased velocity, due to a reduced area, by the continuity law.
- If the velocity is increased, the pressure will be decreased by Bernoulli’s principle.
- A tube connected at the reduced area will create a suction. (principle behind gyro instruments, p. 16)

- If the velocity is increased, the pressure will be decreased by Bernoulli’s principle.

Ahead of the airfoil, the air has a uniform velocity associated with it across the entire stream.

- When it reaches the airfoil, it must divide, with part of the stream flowing over the top surface and part over the bottom surface.

- At this point the cross-sectional area of the stream is reduced by the amount of space the airfoil takes.
- According the the law of continuity, this reduction must cause an increase in velocity.
- The end result is a greater lowering of pressure on the top surface than the bottom, even though the pressure is lowered somewhat over the bottom surface.

- We will begin on airfoil terminology.
- If you have not read all of chapter 2 please do so for next time.

Quiz on Lecture 2

Please take out a sheet of paper.

Include today’s date and your name.

- Please explain Bernoulli’s principle.
- What is the difference in static pressure and dynamic pressure?
- Explain the Law of Continuity.
- How can a venturi tube create lowered pressure?
- How does an airfoil create lift?